1. Field of the Invention
The embodiments of the present invention relate to a method for fabricating an organic thin film transistor (hereinafter, also referred to “OTFT”) by application of an electric field, and more particularly to a method for fabricating an organic thin film transistor which comprises the steps of laminating a gate electrode, a gate insulating layer, an organic semiconductor layer and source/drain electrodes on a substrate, applying a direct current (DC) voltage to between the source and drain electrodes, and applying an alternating current (AC) voltage of various waveforms to the gate electrode.
2. Description of the Related Art
In general, thin film transistors (TFTs) currently used in displays consist mainly of an amorphous silicon semiconductor, a silicon oxide insulating film and metal electrodes. With the recent developments in various electrically conductive organic materials, a great deal of research around the world has focused on developing organic TFTs using organic semiconductors. Since organic thin film transistors (OTFTs) developed in the 1980's have advantages in terms of superior flexibility and ease of processing and fabrication, they are currently used in matrix display devices, such as liquid crystal displays (LCDs). Organic semiconductors as novel electronic materials are increasingly used in a wide variety of applications, e.g., functional electronic and optical devices, on account of their various synthetic processes, easy molding into fibers and films, superior flexibility, and low preparation costs. When compared to silicon transistors using amorphous Si, OTFTs using an organic semiconductor layer made of conductive organic molecules as a semiconductor have advantages in that the semiconductor layer can be formed by printing processes at ambient pressure, instead of conventional chemical vapor deposition (CVD) processes, such as plasma-enhanced chemical vapor deposition (CVD), and optionally, the overall fabrication procedure can be accomplished by roll-to-roll processes using plastic substrates.
Despite these advantages, OTFTs have problems of low charge carrier mobility, high driving voltage and high threshold voltage, when compared to amorphous silicon TFTs. Charge carrier mobility up to 0.6 cm2·V−1·sec−1 has recently been achieved in pentacene-based OTFTs, increasing the possibility of practical applications (N. Jackson, 54th Annual device Research Conference Digest 1996). However, the mobility is still unacceptable for practical TFT applications. In addition, drawbacks of the pentacene-based TFTs are a high driving voltage (≧100V) and a high sub-threshold voltage 50 times than that of amorphous silicon TFTs.
On the other hand, U.S. Pat. No. 5,981,970 and Science (Vol. 283, pp 822-824) disclose organic thin film transistors with reduced driving voltage and threshold voltage using high dielectric constant (κ) insulating films. According to these publications, the gate insulting films are composed of inorganic metal oxides, such as BaxSr1-xTiO3 (barium strontium titanate (BST)), Ta2O5, Y2O3, TiO2, etc., and ferroelectric insulators, such as PbZrxTi1-xO3 (PZT), Bi4Ti3O12, BaMgF4, SrBi2(Ta1-xNbx)2O9, Ba(Zr1-xTix)O3 (BZT), BaTiO3, SrTiO3, Bi4Ti3O12, etc. In addition, the gate insulting films are formed by chemical vapor deposition, physical vapor deposition, sputtering, and sol-gel coating, and have a dielectric constant above 15. The lowest driving voltage of the OTFTs is reduced to −5V, but the highest charge carrier mobility is unsatisfactorily 0.06 cm2·V−1·sec−1. Furthermore, since most of the fabrication steps require a high temperature of 200-400° C., the range of applicable substrates is limited and common wet processes, such as simple coating and printing, cannot be easily applied to fabricate the OTFTs.
U.S. Pat. No. 6,232,157 suggests the use of polyimide, benzocyclobutene, photoacryls and the like as materials for organic insulating films. However, since these organic insulating films exhibit unsatisfactory device characteristics over inorganic insulating films, they are unsuitable to replace inorganic insulating films.
Attempts have been made to use multilayer gate insulating films in order to improve the performance of thin film electronic devices. For instance, U.S. Pat. No. 6,563,174 descries a multilayer gate insulating film consisting of two insulating films made of amorphous silicon nitride and silicon oxide, respectively, and U.S. Pat. No. 6,558,987 describes a double-layered insulating film consisting of two insulating films made of the same material. It is reported that these structures improve the electrical insulating properties and the crystalline quality of semiconductor layers. However, since both gate insulating films were developed only for amorphous silicon- and single crystal silicon-based inorganic TFTs and use inorganic materials, they are not suitable for use in the fabrication of organic semiconductors.
As the application of OTFTs has recently been extended not only to LCD displays but also to driving devices for flexible displays using an organic EL element, the OTFTs are required to have a high charge carrier mobility above 5 cm2·V−1·sec−1, low driving voltage and low threshold voltage, and to exhibit superior electrical insulating properties. Particularly, the fabrication of OTFTs involves a photoresist process for forming a pattern after deposition in order to form source/drain electrodes. At this time, an insulator layer underlying electrodes, in the case of bottom-contact OTFTs, or an organic semiconductor layer underlying electrodes, in the case of top-contact OTFTs, is exposed to a photoresist stripper, and as a result, nitrogen generated from the stripper is adsorbed to the surface of the gate insulating film or organic semiconductor layer, leading to a deterioration in the performance of the transistors.
There have been introduced some trials to reduce the damage to the surface of a gate insulating film. For example, the damaged surface of a gate insulating film is physically treated using inert gas plasma, or a self-assembled monolayer (SAM) is formed on the damaged gate insulating film. However, these trials are not efficient in that they need additional processes, e.g., a process for physically treating the damaged surface.
Thus, there is an urgent need to develop a method for fabricating an OTFT which enables recovery of damage caused during fabrication of an OTFT by simple treatment, thereby not only ensuring high charge carrier mobility, low driving voltage and low threshold voltage but also achieving formation an insulating film by common wet processes.
The present inventors have earnestly and intensively conducted research to satisfy the need, and as a result, have found that when an electric field is simply applied to source, drain and gate electrodes during fabrication of an OTFT using a polymer semiconductor by wet processes, the charge carrier mobility and on/off current ratio (Ion/Ioff ratio) of the OTFT are greatly improved, leading to an improvement in the performance of the OTFT. Embodiments of the present invention are based on this finding.